The most common implementation of automotive automatic transmissions is what is known as torque converter transmissions, which differ from conventional and automated manual transmissions primarily through a shifting process that does not interrupt tractive force. Such automotive automatic transmissions generally use a hydraulic torque converter as the starting clutch, consisting of an impeller, a stator, and a turbine wheel, and are composed primarily of a varying number of multi-plate clutches and a combination of multiple planetary gear sets. All shifting and clutch operations in this design take place by means of various one-way clutches and with the multi-plate clutches, which establish the frictional connection of the individual planetary gear set stages with the input and output shafts of the automatic transmission. A gear change thus takes place through gradual uncoupling of one control element consisting of a clutch and a shaft of a planetary gear set, and simultaneous coupling of a second control element for the next gear consisting of another clutch and another shaft of a planetary gear set, until the full torque of the first control element is taken on by the second control element at the end of the gear change.
Because substantial axial forces also act in the hydraulic torque converter and, during the individual shifting operations, between the multi-plate clutches and the planetary gear sets, in modern automatic transmissions the impeller and the turbine wheel, as well as the individual clutch carriers and planet carriers, are braced against one another by up to 17 axial needle bearings depending on the number of gears, in order to avoid frictional and efficiency losses. Axial needle bearings of this nature are known from the applicant's “Roller Bearings” catalog, January 2006, on pages 776 to 782, under the product designation AX, and consist essentially of a first annular bearing disk and a second annular bearing disk that are each made of a thin-walled sheet steel and are arranged at a distance from one another on a common center axis. The axial inner sides of the two bearing disks are implemented as races, and between them roll a plurality of bearing needles arranged next to one another, which are held at uniform distances from one another by a bearing cage; the bearing needles and bearing cage together form a needle roller and cage assembly.
However, under continuous operating conditions of such automatic transmissions, it has been shown that the axial needle bearings used still cause frictional losses that are a contributing cause of a reduction in the desired efficiency of the automatic transmission, and the reduction in said losses thus offers considerable potential, particularly against the background of present demands for reduction of CO2 emissions from motor vehicles with internal combustion engines. The reduction in efficiency here can primarily be attributed to the frequent change of the axial needle bearings between the loaded and unloaded states resulting from the individual shifting operations in the automatic transmission. In this context, the axial needle bearings have high frictional torques in the loaded state that result from the line contact of the rolling elements to the races of the bearing disks and from the rolling element slip caused by the different rotational speeds at the inner and outer circles of the rolling elements. Although the frictional torque resulting from the line contact of the rolling elements with their races is reduced in the unloaded state of the axial roller bearings, the aforementioned rolling element slip increases in this state to such a degree that a kinematically undesirable rolling of the rolling elements on the races takes place to the point where the rolling elements or the entire needle cage comes to a standstill, with the rolling elements merely sliding over their races and thus producing a torque of similar magnitude to the loaded state of the axial roller bearing. Moreover, when sudden loading of the axial roller bearing and the associated abrupt acceleration of the needle roller and cage assembly take place, so-called smearing of the rolling elements on their races occurs, which in addition to frictional heating of the rolling elements causes increased wear of the races and is ultimately responsible for shortened service life of the axial needle bearings.
In order to avoid the disadvantages resulting from rolling element slip in axial roller bearings, DE 199 24 018 A1 proposed designing the races of the rolling elements with an inward curvature over their entire width, but such a measure was not able to attain the desired effect, or to attain it in full, since the axial deflection stiffness of bearing rings designed in this way has proven to still be too high, and the rolling element slip described above continues to occur in certain intermediate load regions; this slip is the cause of continuously high frictional torques in both the loaded and unloaded states of the axial roller bearings.